SUJET : Mail not sent on send email invitation

I have set up mail system on LSurvey and users/admins received emails upon setup process.
When I add participants on survey, invitation email didn't shoot out.
I got this error message below & nothing is missing.

Warning

There were no eligible emails to send. This will be because none satisfied the criteria of:

having a valid email address
not having been sent an invitation already
having already completed the survey
having a token

edit: LSurvey version is 2.00+ Build 121204
Any thoughts on that!
Thanks